Stable www/firefox35 couldn't fetch "firefox-3.5.10.source.tar.bz2" from locations specified in Makefile. I was able to download it from ftp://ftp.mozilla.org/pub/firefox/releases/3.5.10/source/ to distfiles and 'make install'ed OK.
Seems like they move older files after a certain time... Best regards, Alex Libman -- http://www.fastmail.fm - A fast, anti-spam email service.